AutoCAD 2007 is /3GB switch aware if you make the boot.ini file changes recommended by Microsoft (see link below). Of course the only gain is you can load larger models into memory but there is no real performance gains. A strong word of caution is you could effect other applications on your system that are not "LargeAddressAware" with this OS boot.ini switch.
http://www.microsoft.com/whdc/system/platform/server/PAE/PAEmem.mspx